1. Field of the Invention
The invention relates to an exhaust device for a vehicle internal combustion engine. In particular, the invention relates to an exhaust device for a vehicle internal combustion engine that emits the exhaust gas discharged from a multi-cylinder internal combustion engine mounted in a vehicle to the outside after the exhaust gas is divided and passed through at least two silencers.
2. Description of the Related Art
Lately, exhaust devices for vehicle internal combustion engines adopt a so-called duel-pipe exhaust system in which the exhaust pipe is branched at a downstream side into left and right branch pipes, and each of the branch pipes is equipped with a silencer, and exhaust gas is emitted from left and right exhaust outlets in a rear portion of the vehicle, since the system facilitates the enlargement of the volumes of the silencers, and is excellent in design characteristic.
In a known related-art exhaust device of this kind for a vehicle internal combustion engine, for example, the exhaust pipes extending from the two banks of a V-type engine are merged into a single pipe that extends through a catalyst device and a pre-muffler, and the exhaust gas having passed through the pre-muffler is divided in flow and led into left and right rear mufflers via branch pipes, whereby the exhaust gas is silenced (see, e.g., Japanese Patent Application Publication No. 2005-248767 (JP-A-2005-248767)).
Besides, in another known exhaust device, in order to shut off roll vibration during operation of the engine, an upstream-side portion of each of the left and right exhaust paths is equipped with a ball joint (spherical pipe joint) that permits the immediately upstream and downstream exhaust pipes to oscillate relatively to each other, and a flexible pipe is attached immediately downstream of one of the left and right ball joints so as to absorb vibration displacements other than the permitted oscillations (see, e.g., Japanese Patent Application Publication No. 09-317461 (JP-A-09-317461)).
However, in the related-art exhaust devices for vehicle internal combustion engines as described above in which a downstream-side portion of the exhaust pipe is divided and led via branch pipes to the left and right rear mufflers for silencing exhaust gas, the left and right branch pipes (branching pipes) are provided in substantially the same shape, taking into account the left-right exhaust balance (equal amounts of exhaust gas, and similar exhaust sounds). Therefore, the left and right rear mufflers and branch pipes having relatively large masses tend to vibrate in such phases that the vibrations thereof overlap each other at the branching point between the branch pipes. Therefore, in a specific vibration frequency range, for example, in the idle vibration range, the vibrations of the left and right rear mufflers and branch pipes in substantially the same phase strengthen each other, and then are transmitted to the vehicle body side, resulting in a problem of vibration unpleasant to an occupant occurring in a cabin floor or the like near the driver's seat. In particular, in the case where the front pipes connected to the front and rear exhaust manifolds have unequal lengths as is the case with a transversely mounted V-type engine, there are many constrains regarding the arrangement of ball joints at the downstream end side of the front pipes, and transmission of vibrations from the engine easily occurs, thus giving rise to the following problem. That is, vibration tends to occur in the floor beneath the driver's feet, in the idle vibration range of, for example, 30 Hz to 40 Hz in the case of a six-cylinder engine, and the vibration becomes conspicuous due to the vibration transmitted from the side of the left and right rear mufflers and branch pipes.